Verizon worked with officials at Northern Illinois University yesterday to bring in two trucks full of equipment to try to handle a burst of cellphone calls on and around the campus. Many circuits were busy yesterday as students called home to say they were safe after a gunman opened fire in a classroom.

The cellphone trucks are known as COWs, for Cellular antennas on Wheels. Walter L. Czerniak, associate vice president for information-technology services at Northern Illinois, said the trucks were to arrive on the campus this morning. The plan was for one to be used immediately, and for the second to be activated if necessary.

Each truck can support an additional 250 to 500 simultaneous cellphone calls, Mr. Czerniak said. He did not know whether other cellphone companies had added equipment as well, but he said that Verizon had a close relationship with Northern Illinois because it has a contract with the state university system to provide telephone service to the campus.

Even though the immediate danger has passed, Mr. Czerniak said, the campus is still seeing an increase in cellphone call volume today. —Jeffrey R. Young